In Transition from Illegal Regimes in International Law, which is the commercial version of a PhD thesis defended at Cambridge University, Yaël Ronen examines the legal and political constraints which post-transition regimes face when pondering a return to the law applicable before a prior illegal regime came to power. Examples, also discussed at length in the book, are the Baltic States' contemplated policy of erasing the Soviet Union's imprint on their legal systems during the period of—illegal—annexation between 1940 and 1990, and Timor-Leste's struggle to come to terms with the legal changes brought about by Indonesia's—illegal—presence between 1975 and 1999.

Ronen does not disappoint. In a meticulously researched and very well-written book, she focuses on six case studies, and analyses in particular, also from a doctrinal perspective, the effect of transition from an illegal regime on treaty relations of the territory, the domestic law of the territory, settlers implanted by illegal regimes, and land titles. The selection of the case studies is defensible, and the cases are painstakingly investigated in minute detail, including on the basis of field visits and interviews where legal texts and existing analysis were lacking, eg in respect of the artificially created South African Bantustans. As a minor point, it surprises that Israel's annexation of the Golan Heights and the occupation of Gaza and the West Bank, accompanied by the implantation of a large number of settlers, is not addressed. The fact that the current status of these territorial entities resulted from Israel's exercise of self-defence, that there is little actual discussion of transition with regard to these entities, and that the law of occupation is considered a more appropriate legal framework, does not seem to be fully convincing. This issue, nonetheless, does not cast a shadow over the structure and the analytical quality of the work.

Ronen starts her doctrinal analysis by reminding the college of international lawyers of the obligation of non-recognition applicable to illegal territorial regimes—regimes that came into being as a result of a violation of a peremptory norm, eg through annexation—and their acts. What international lawyers are generally less familiar with is whether this obligation is, or should be, seen through to its logical conclusion; namely, that all acts adopted by the illegal regime should not only be considered invalid ab initio but should also not be given any practical effect, including after the demise of the illegal regime. It goes to Yaël Ronen's credit that she has taken seriously the ‘facts on the ground’ that have been created by the often prolonged existence of an illegal regime, and which individuals may legitimately have relied upon. To be sure, in its Namibia Advisory Opinion, the International Court of Justice had already observed that the scope of obligations of non-recognition is not absolute, and that non-recognition should not go to the detriment of the interests of the local population. The classic standard employed in this respect is whether acts ‘entrench’ the illegal regime; if they do, they should not be given any legal effect, including after transition. Ronen, for her part, suggests an alternative, case-by-case approach informed by international human rights law. The result of such an approach is that the post-transition regime's political objective of returning to the status quo ante, which wipes out the consequences of the previous regime's presumptively illegal acts, may be interfered with by long-term residents'(including settlers) rights to housing, family, and private life.

In doing so, Ronen shows that, given the acquired rights and legitimate expectations of innocent individuals caught up in geopolitical events, transition from illegal regimes is necessarily messy, and reliance on principle may not be particularly helpful to bring closure or justice to affected individuals. Somewhat counterintuitively, and to some possibly shockingly, this means that the illegal regime may survive its own demise, and that the principle of non-recognition, which is supposed to enforce a return to legality, will have little practical effect. This is a sobering observation, but it is informed by justified considerations of humanity: should one indeed allow eviction of long-term tenants of property in favour of relatives of previous owners who, decades ago, have been dispossessed by the illegal regime, and may have found alternative housing and sources of income in the meantime? These are essentially moral questions, which inform the criteria of a legal test that balances the post-transition regime's political desire to enforce the invalidity of the illegal regime's acts with individual rights.

I have to admit that I found the lengthy discussion of the case studies in the different legal areas, while necessary for a clearer grasp of the issues, to be rather technical and tedious at times. Moreover, some discussions may not withstand the test of time, such as the Annan Plan for Cyprus, which was rejected in a referendum, and was replaced by an ‘Obama’ plan in 2014. Having gone through the cases, I am also wondering whether the policies applied by the post-transition regime, in respect to the relevant territories, were really informed by international law considerations, or just by political pragmatism instead. To be sure, some patterns may be discernible (which are very intelligently discussed in the concluding part of the book), such as the upholding of the invalidity of treaties concluded by the illegal regimes and the wide-ranging validation of domestic legislation adopted by such regimes. However, it remains unclear whether these patterns also coalesce into legal principles that can sufficiently guide future practice, eg in case the Crimea—illegally annexed by the Russian Federation—ever returns to Ukraine. Nonetheless, Yaël Ronen should not be blamed for that. Ultimately, transitions from illegal regimes are so fraught by political sensitivities that the integrity of the law inevitably takes a back seat.